Flushed with triumph after its production of The Mountaintop won an Olivier best new play award, this enterprising Battersea theatre lets its hair down with a musical. I've no objection to that, and the piece is performed with spirit. But this transformation of an hour-long Edinburgh fringe hit into a two-and-a-half-hour show is, for all the titillation of its title, surprisingly hard-going and suggests less might have been more.

The premise is simple enough: Stefan, a Maltese Candide, emigrates to LA, where he hopes to become a famous carpenter "like Harrison Ford or Jesus". Mugged on arrival, he is rescued by a porn princess who, in a witty send-up of a country and western number, announces: "My name is Sanddy with a double D." The big-hearted Sanddy initiates Stefan into the adult-movie business, where, under the direction of one Martin Scoresleazy, he proves a fast-rising star. But the blossoming romance between Sanddy and Stefan is subject to sundry vicissitudes ranging from Martin's financial desperation to sexually transmitted diseases.

As a musical, it has its moments. The best bit comes when it satirises the double entendres of porn-movie dialogue: crying out for a handyman, Sanddy's heroine claims: "I've got something here that urgently needs a good hammering." But the basic problem is that the show's creators – Boris Cezek, Kris Spiteri, Malcolm Galea and Abigail Guan – don't have any definable attitude to their subject: they seem undecided whether porn movies fulfil a vital function or offer a degradation of sex. They treat it all as a lark and an excuse to send up musical conventions. The attempt to show that true love can flourish in the unlikeliest surroundings also leads to improbable lyrics ("I'm sorry that I hurt you and I gave you pubic lice").

Yet again, one is reminded that a musical is only as good as its book – it has to make you care about something. Here the tone is one of pervasive jokiness, partially redeemed by Paul Robinson's energetic production. There are likable performances from Brendan Cull as the naive hero, Sophia Thierens as the sweet-natured Sanddy and David Burt as their rancidly corrupt director. The tunes by Cezek and Spiteri slip down easily, but the show fails to take its subject seriously and seems unsure whether to expose or exploit the porn brokers. In the end, whatever the show's silhouetted action may suggest, you can't have it both ways.